Contingency table and chi-square test for gene direction/replication orientation and sign of skew value.

All mitochondrial genomes used in this study were included in analysis. R2P3, two-fold redundant third codon positions; R4P3, four-fold redundant third codon positions; P, significant value; X2, Chi-square value; RO×AT, replication orientation and sign of AT skew; RO×GC, replication orientation and sign of GC skew; GD×AT, gene direction and sign of AT skew; GD×GC, gene direction and sign of GC skew.